1. Field of the Invention
This invention relates generally to a communication system, and, more particularly, to a wireless communication system.
2. Description of the Related Art
In conventional wireless telecommunications, one or more access terminals may establish a wireless link to a Radio Access Network (RAN). The RAN architecture is typically hierarchical and call state information associated with each access terminal call session is stored in a central repository, such as a Radio Network Controller (RNC), a Packet Data Serving Node (PDSN), and the like. One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of base station routers. For example, each base station router may combine RNC and/or PDSN functions in a single entity that manages radio links between one or more access terminals and an outside network, such as the Internet. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the RNC and PDSN of hierarchical networks may be reduced or removed.
In a distributed architecture, the base station router incorporates the RNC and PDSN functionality. One or more network access agents associated with access terminals in the service area of the base station router may implement Mobile IP (MIP). The network access agents are typically a part of the PDSN protocol suite and are responsible for providing connectivity between the access terminal and the distributed network. For example, the network access agent may be a foreign agent that provides Point of Attachment (PoA) and/or Care of Address (CoA) functionality for the access terminal. The access terminal also registers with a home agent (HA) that provides a permanent/semi-permanent attachment to an Internet Protocol (IP) network and maintains information that identifies the foreign agent that is serving the access terminal. Accordingly, information transmitted by the access terminal may travel over an air interface to the base station router and then be directed to the IP network via the home agent. Information intended for the access terminal may travel from the IP network to the base station router via the home agent and then to the access terminal over the air interface.
The base station router that provides the air interface to the access terminal may change if the access terminal moves through the wireless network. For example, if an access terminal having an active call session moves from a geographic area served by a first base station router to a different geographic area that is served by a second base station router, then the network access agent associated with the access terminal may be migrated or transferred from the first base station router to the second base station router, and the second base station router then becomes the serving base station router. The home agent typically remains the same as the access terminal roams throughout the network. If the access terminal moves frequently, then the additional overhead associated with repeatedly transferring the network access agent may undesirably consume scarce network resources.
When an access terminal becomes idle or dormant, the air interface with the serving base station router and any data paths in the radio access network are torn down to conserve scarce network resources. Migrating the network access agent associated with a roaming idle access terminal also consumes scarce network resources. For example, a foreign agent may relay traffic between the home agent and the access terminal according to Mobile IP techniques, in which case the foreign agent may be referred to as a layer 3 anchor. Transferring a layer 3 anchor from one base station router to another base station router requires forming a new radio connection to the access terminal, which adds signaling overhead and brings the access terminal back to the active mode. If the idle access terminal moves frequently, then the additional overhead associated with repeatedly transferring the layer 3 anchor may consume scarce network resources, potentially defeating the purpose of placing the access terminal in the idle mode. Consequently, conventional distributed networks do not typically migrate foreign agents associated with idle access terminals until the access terminal is activated in response to incoming or outgoing communications.
However, failing to migrate the foreign agent associated with idle access terminals may also have a number of drawbacks. Depending on the circumstances, the idle access terminal may roam to a geographic area associated with a base station router that is distant from the previous serving base station router. Thus, when the access terminal wakes up and attempts to resume the idle or dormant call session, the foreign agent may need to be migrated to the new base station router. For example, the access terminal may need to re-register with the network and may need to perform a foreign agent relocation to the new base station router before an active communication link can be established with the base station router. The foreign agent relocation procedure may increase the session reactivation delay for the access terminal. The session reactivation delay may be particularly troublesome if the call session associated with the access terminal is designed to respond rapidly to incoming and/or outgoing information. For example, Push-to-Talk applications are designed to respond substantially immediately to incoming and/or outgoing information and so may not operate in the desired manner when the session reactivation delay is significant.